1. Field of the Invention
The present invention pertains to electromagnetic valve actuator systems. More particularly, the invention pertains to an electromagnetic valve actuator system that opens and closes the poppet valves of an internal combustion engine.
2. Description of Related Art
Conventionally, valve trains of internal combustion engines include poppet valves that are spring loaded toward a valve-closed position. The poppet valves are biased open either by an overhead camshaft mechanism or by a cam and push rod mechanism. In either case, the camshaft is connected to and rotates in synchronization with an engine crankshaft to open and close each valve at predetermined intervals as defined by the position of lobes on the camshaft. Therefore, the sequence and lift distance of each valve is fixed by the position and size of the lobes on the camshaft, and the frequency of the operation of each valve is proportional to engine crankshaft speed.
Such direct-drive arrangements fix valve train operation and thereby limits engine performance because ideal valve timing varies, and is not fixed, over the full range of engine speed. Therefore, it would be desirable to incorporate an indirect drive arrangement in which the valve train is not fixed, but is independently variable with respect to each valve. Such factors as lift distance, lift speed, and seating velocity could be varied independently for each valve. These factors can be varied to improve breathing of the engine to increase performance, fuel economy, or to reduce emissions. The variable cam timing (VCT) devices of the prior art allow for variable phasing of the valve train with respect to engine crankshaft speed, but do not allow for independent variability of the valves.
Because of the above-described limitation of VCT devices, many inventors have abandoned the direct drive and VCT architectures in favor of electromagnetic valve actuator systems. Such systems have the potential to increase overall engine efficiency by reducing frictional losses associated with the conventional valve train, and by reducing heavy components such as the camshaft, chain, sprockets, and VCT devices. Such systems are also capable of closing certain valves to permit the engine to operate as a “smaller”, more efficient, engine under high speed/low torque conditions. Unfortunately, however, these electromagnetic valve trains have not gained widespread acceptance in the marketplace, primarily due to a substantial increase in part count, poor valve seating reliability, and increased noise, vibration, and harshness (NVH) during operation.
These actuator systems use flat disk-like armatures that are positively secured to the valve and are axially trapped between ring-like tractive electromagnets. The electromagnets have poles at one end that attract the armature to either an open or closed position against the respective poles of the electromagnets. Unfortunately, as the valve heats up under normal operating conditions, the valve expands in length and does not have a chance to seat before the armature stops against the respective pole. Additionally, increased NVH (noise, vibration, and hardness) results from the valve and armature colliding against their respective mating surfaces. This results because the force on the armature increases cubically as the distance between the armature and the pole decreases. Therefore, the armature is accelerating as it approaches the pole, and the force on the armature is at a maximum just as the armature makes contact with the pole.
A review of the prior art yields scores of electromagnetically actuator valve devices directed at remedying valve seating problems and NVH during operation. For example, U.S. Pat. No. 4,455,543 (Pischinger et al.) and U.S. Pat. No. 4,749,167 (Gottschall) use spring systems attached to electromagnet armatures to decelerate a valve to the full open or closed position. U.S. Pat. No. 4,515,343 (Pischinger et al.) uses a bellows device mounted coaxially within an electromagnetic actuator to adjust the distance between the valve seat and the electromagnet pole so that it corresponds to the distance between the valve head and the electromagnet armature, so that a desired amount of dampening is consistently achieved. U.S. Pat. No. 5,878,704 (Schebrtz et al.) uses a sound muffling layer sandwiched between the electromagnets of the electromagnetic actuator to absorb vibration from the armature slapping against the poles of the electromagnets. U.S. Pat. No. 5,592,905 (Bam) replaces heavy iron armatures with a lightweight conductive armature that is finely controlled by varying current supplied to the armature. U.S. Pat. No. 5,647,311 (Liang et al.) and U.S. Pat. No. 6,003,481 (Pischinger et al.) each use at least one auxiliary electromagnet and armature to provide additional control of the closing force of the valve. Finally, U.S. Pat. No. 5,636,601 (Moriya et al.), U.S. Pat. No. 5,671,705 (Natsumoto et al.), and U.S. Pat. No. 6,016,778 (Koch) use control circuits to vary the current supplied to the electromagnets in accordance with varying operational temperatures to gain a more controlled seating of the valve.
All of the above-listed references have significant disadvantages that render their use unlikely in the marketplace. First, some are limited to a single valve lift distance, and thus do not fully take advantage of potential engine efficiencies and operate on a fixed disk-like armature that may seat against the electromagnet pole before the valve head seats with the valve seat. Others involve expensive additional components such as bellows, current carrying armatures, muffling devices, and additional electromagnets and armatures. Finally, others incorporate complex control circuits to bandage the inherent hardware problems of the prior art. Such control schemes face the difficult task in reducing current to the electromagnet fast enough to slow the accelerating armature.